This one allows for complex shaders paired with a simple lighting shader to skip code that would otherwise be pointlessly (and wastefully) run during the light pass.
You can use `if (AT_LIGHT_PASS) , negated or not, and that will be converted to a preprocessed #if when the shader is compiled.
Depending on your game (number of items and lights), this can be a *significant* performance gain, or at least avoids relying on the driver's optimizing abilities.
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